Straying very slightly away from the "asking to change" twice i have boarded and been in an A or F seat (not front row) and someone was sitting in it. When i asked them the seat number, it turned out they were the opposite side of the plane, same row, still at window, but just had got mixed up.

Without even thinking about it, i said it was ok and that I would just sit on their seat as there was no point in disturbing everybody.

After the 2nd time of this happening, i have noticed it happening more frequently(not with me) and i scratch my head when people dont take the simple option.

I know people may say, for certain not to be mentioned reasons, you should sit in your assigned seat, but is it really that important if its like for like on either side of the same row ?